Light-based technologies are extensively researched, with some of them already in commercial use, in various fields including communications, measurement, and fabrication to exploit their many advantages in frequency (speed), capability to handle phase and space data processing, etc.
The technology depends on a highly precise objective lens to focus a light beam. The highly precise objective lens is playing an increasingly important role in recent development of bulk storage technology, due to an outstanding demand for light-based, image-capturing apparatus.
To develop an optical bulk data storage technology, or specifically, to develop storage media with an increased recording density, the objective lens needs to be capable of producing a sufficiently focused beam spot on the recording face of a storage medium. As conventionally known, the size of the beam spot produced by an objective lens is directly proportional to the wavelength of light and inversely proportional to the NA (Numerical Aperture) of the objective lens.
As to reduction in light beam wavelength, progress has been made as a result of recent development of blue laser diodes and blue or green SHG lasers.
The NA of the objective lens is also improving. An example is the DVD (Digital Versatile Disc) which boasts an NA of 0.6, in view of the NA of the CD (Compact Disc) as low as 0.45.
Thus, studies are going on to increase the recording density of the storage medium in both aspects: reduction in light beam wavelength and growth in objective lens's NA.
Japanese Unexamined Patent Application No. 10-123410/1998 (Tokukaihei 10-123410; published on May 15, 1998) discloses an optical pickup arranged from two objective lenses, having an NA of 0.85, which are positioned opposite to each other with a predetermined distance therebetween, to achieve a higher density.
Due to the high NA of these two lenses arranged in two separate groups (hereinafter, “double-group lenses”), adjustment needs to be made to compensate for irregularities in thickness of transmissive layers in a storage medium and correct spherical aberrations which occur in the process of multilayer recording. For example, Japanese Unexamined Patent Application No. 10-142494/1998 (Tokukaihei 10-142494; published on May 29, 1998) discloses a technique to correct spherical aberrations by varying the distance between the objective lens. Japanese Unexamined Patent Application No. 2000-131603 (Tokukai 2000-131603; published on May 12, 2000) is another example, disclosing a technique to correct spherical aberrations of an objective lens group 201 composed of lenses 201a, 201b by altering along the optical axis OA the gap between two lenses 202a, 202b constituting an aberration-correcting optical system 202 which is positioned opposite to a storage medium 200 across the objective lens group 201 as shown in FIG. 27.
However, the use of double-group lenses as an objective lens in an optical pickup give rise to following problems.
Double-group lenses are typically housed in a barrel to maintain a predetermined distance between two lens. For example, the double-group lenses disclosed in Japanese Unexamined Patent Application No. 10-123410/1998 introduced above should be positioned elaborately through complex steps when placed in a barrel or similar object to form a single entity. The positioning should be highly precise as to the lens-to-lens distance and the inclination and displacement of the lenses with respect to an optical axis and is conducted by actually transmitting a laser beam through the lenses to observe a focused beam spot and aberrations.
Resin which secures the double-group lenses to the barrel is by no means reliable, because it lacks sufficient resistance to heat and humidity and is likely to deteriorate with time and for other factors.
The optical pickup has a mechanism including a driver, termed actuator, which moves the objective lenses independently in directions both parallel and perpendicular to the optical axis for focusing and tracking purposes. The double-group objective lenses, including the barrel which holds them together, become too heavy to move at high speed, which presents an obstacle in the pursuit of high speed reading and writing.
A possible alternative is the use of a single lens or multiple lenses cemented together with no intervening air space therebetween (hereinafter, collectively called “single-group lens”) as an objective lens. The use of these kinds of lenses can address the aforementioned problems caused by the use of double-group lenses as an objective lens as well as provide a means to fabricate a lightweight, highly reliable pickup.
A problem in the use of a single-group objective lens is that the lens is subject to property deterioration due to coma aberration if the NA has a large value. Coma aberration occurs from manufacturing tolerances: errors in surface-to-surface distance, surface precision, surface tilts (inclinations), and a shift (decentration) of the objective lens. Specifically, when the NA is as low as about 0.6, the objective lens is not affected by coma aberration.
Conversely, when the single-group objective lens has a NA as high as that of the foregoing double-group objective lenses (for example, 0.85), the single-group objective lens experiences serious property deterioration due to coma aberrations caused by manufacturing tolerances.
Coma aberration is also caused by inclination of the single-group objective lens to the recording surface of the optical storage medium.